1. Field of the Invention
The present invention relates to a method of non-destructively inspecting internal defects such as microcracks or other defects which may occur in a piezoelectric ceramic device such as an oscillator, a filter, or other such device, which defects affect the qualities and characteristics of the piezoelectric ceramic device.
2. Description of the Related Art
In a conventional method of non-destructively inspecting an internal defect of a piezoelectric ceramic device, the impedance and/or phase characteristic of a piezoelectric ceramic device is measured, the curve pattern representing the characteristic is compared with a standard curve pattern, and if the curve patterns are different from each other, it is judged that microcracks are present in the piezoelectric ceramic substrate, as described in Japanese Unexamined Patent Application Publication No. 6-3305.
According to such an inspection method, automatic judgement is possible. Thus, advantageously, judgement as to whether lots of piezoelectric ceramic devices are non-defective or defective can be performed quickly and efficiently. Moreover, the inspection accuracy is high, since the inspection is not visually performed.
According to the above-described inspection method, an electrical characteristic is measured at an ordinary temperature and compared with a standard characteristic. However, there are many cases in which a difference between non-defective components and defective components is small or is non-existent at ordinary temperatures. Thus, using the above-described inspection method, it is impossible to detect an internal defect such as microcracks or other defects completely.
In one example, each of a plurality of ceramic oscillators (oscillation frequency: 25 MHz) was incorporated in an oscillation circuit, the oscillator was placed in an atmosphere at a temperature of 200xc2x0 C. while the oscillator was being oscillated, and then, characteristics of the oscillation voltage were measured. FIG. 1 shows the measurement results. The characteristics of these oscillators are not different at ordinary temperature.
As seen in FIG. 1, the oscillation voltages are slightly decreased with the increase of the temperature. Some of the oscillators (NG) are decreased to about OV, and the oscillation is stopped. For the other oscillators (G), the oscillation is not stopped even at 200xc2x0 C. or higher.
These oscillators were opened, and the internal devices were observed with a microscope. For the oscillators (NG) that had their oscillation stopped at a low temperature, it was determined that microcracks were formed inside of the devices. Accordingly, it has been discovered that some oscillators that normally oscillate and exhibit normal characteristics at ordinary temperatures have microcracks inside thereof, and such an internal defect can be detected by measuring the characteristic of oscillators while they are heated according to preferred embodiments of the present invention described below.
In order to overcome the problems described above, preferred embodiments of the present invention provide a method of inspecting a piezoelectric ceramic device in which an internal defect, which may be undetectable at ordinary temperature, can be accurately and reliably detected non-destructively.
In addition, preferred embodiments of the present invention provide a method of inspecting a piezoelectric ceramic device in which an internal defect can be detected at high speed using a simple instrument.
According to a preferred embodiment of the present invention, a method of inspecting a piezoelectric ceramic device includes the steps of heating and increasing the temperature of a piezoelectric ceramic device to an increased temperature that is in the vicinity of a maximum temperature at which the piezoelectric ceramic device, when the temperature of the device is returned to ordinary temperature, is returned to substantially the same piezoelectric characteristic as that before heating, measuring at least one of the piezoelectric phase characteristic and the impedance characteristic of the piezoelectric ceramic device while the device is heated and the temperature thereof is increased, comparing at least one of the measured piezoelectric phase characteristic and the measured impedance characteristic with a standard characteristic, and detecting the presence or absence of an internal defect of the piezoelectric ceramic device based on results of the step of comparing.
In this preferred embodiment of the present invention, first, the piezoelectric ceramic device is heated so that the temperature thereof is increased.
Next, when at least one of the phase characteristic and the impedance characteristic of the piezoelectric ceramic device in a state of heating and increasing the temperature, a large change is shown with the piezoelectric ceramic device having internal defects in accordance with the temperature increase, such large change is not shown in the normal temperature. Then, one of the measured phase characteristic and the measured impedance characteristic is compared with the standard characteristic. The standard characteristic may be attained from the phase characteristic or the impedance characteristic of the good piezoelectric ceramic device (without internal defects), for example.
As a result of the above-described comparison, if the measured characteristic is different from the standard characteristic so as to exceed a predetermined range of the characteristic, it is judged that the piezoelectric ceramic device has an internal defect.
In addition, according to preferred embodiments of the present invention, not only microcracks but also foreign matters stuck to an electrode can be detected.
It would be preferable to set the increased temperature of heating to the temperature which is in the vicinity of the maximum temperature in which the piezoelectric characteristic of the piezoelectric ceramic device returns to substantially the same as that of before heating when the piezoelectric ceramic device is back to the normal temperature after heating. The internal defects, which cannot be inspected in the normal temperature, can be securely inspected by heating at as high temperature as possible as long as the piezoelectric characteristic can come back. When the piezoelectric ceramic device is heated at higher temperature than the above described temperature, the piezoelectric characteristic of the piezoelectric ceramic device itself is changed non-reversibly, thereby being not preferable.
Preferably, as the phase characteristic to be measured, a maximum phase angle xcex8max is used. At ordinary temperature, the piezoelectric ceramic device presents the phase characteristic shown by solid line P1 in FIG. 2. At a higher temperature, the phase is reduced as shown by broken line P2. The larger the internal defect of the piezoelectric ceramic device becomes, the more the amount of phase reduction increases. Preferably, an internal defect is judged by utilization of the phase reduction.
FIG. 3 shows the results obtained when devices NG having an internal defect and devices G having no internal defect are heated in the same manner, and the maximum phase angles in the vicinity of the oscillation frequency fosc are measured. As seen in FIG. 3, the devices NG having an internal defect present a larger reduction in maximum phase angle, as compared with the devices G having no internal defect. Thus, it can be seen that there is a correlation between the internal defect and the maximum phase angle.
For the devices NG having an internal defect, the phases are not more than about 60xc2x0. On the other hand, for the devices G having no internal defect, the phases are not less than about 70xc2x0. In the case of these devices, it can be securely detected whether an internal defect is present or not by setting the raised temperature for use in judgement of a non-defective or a defective at about 150xc2x0 or higher, and moreover, setting the maximum phase angle as a threshold for the judgement of whether a device is non-defective or defective.
Preferably, as the impedance characteristic to be measured, the difference Zaxe2x88x92Zr between an antiresonance impedance Za and a resonance impedance Zr is used. At ordinary temperature, the impedance characteristic can be shown as indicated by solid line I1 in FIG. 2. At a higher temperature, both of the antiresonance and resonance points are shifted to the higher frequency side, and moreover, the impedance difference Zaxe2x88x92Zr is reduced. Preferably, an internal defect is judged by use of this characteristic.
In addition to Zaxe2x88x92Zr, values per se of Za and Zr, a frequency change ratio (dZa/df) at an antifrequency point, a frequency change ratio (dzr/df) at a resonance point, an oscillation frequency fosc, an anti-resonance frequency fa, and a resonance frequency fr may be used.
It should be noted that the phase characteristic and the impedance characteristic to be measured are not limited to the above ones. Other suitable measurements may also be used.
Referring to a method of heating the piezoelectric ceramic device to increase the temperature, for example, the device is placed in a high temperature atmosphere, or externally heated via a heater. However, according to these methods, it takes at least several seconds to increase the temperature of the device. The efficiency is very low, and moreover, large-scale equipment is required for heating. Accordingly, preferably, the heating and temperature-raising is performed by that a high frequency measurement signal having a level that is higher than the rated level of the piezoelectric ceramic device is applied to the piezoelectric ceramic device with the piezoelectric ceramic device itself being dielectric-heated via application of the high frequency signal, and the measuring step is performed by that at least one of the phase characteristic and the impedance characteristic of the piezoelectric ceramic device in accordance with the application of the high frequency signal is measured.
That is, the inventors of the present invention discovered that by increasing the power level of the measurement signal, the device is quickly heated via dielectric heating. In this case, if the power level is excessively high, the temperature becomes too high, resulting in deterioration of the characteristics of the device. In such a case of overheating, the piezoelectric ceramic device, when the temperature is returned to ordinary temperature, cannot be returned to the piezoelectric characteristic before heating. For this reason, it is preferable to select a high frequency measurement signal having a level that is higher than the rated level of the device and is the possible highest level in the range of temperature where the piezoelectric characteristic can be returned to the characteristic existing before the device was heated. Moreover, devices having different oscillation frequencies have different thicknesses. Accordingly, it is necessary to select the power level corresponding to such differences.
As described above, the heating and temperature-increasing step and the measuring step can be performed simultaneously using the same equipment. As a result, the heating and measuring time is greatly reduced. Moreover, it is only necessary to increase and control the power level of an existing measuring instrument. Thus, these steps can be performed using very simple and relatively inexpensive equipment.
FIG. 4 shows increasing-temperature curves (calculation values) obtained by heating the device by dielectric heating. The respective curves are obtained by changing stepwise the power level from about 30 dBm to about 40 dBm. As seen in FIG. 4, when a time period of about 400 msec. elapses from the start of application of heat, the temperature nearly reaches the maximum temperature.
As described above, when the device is heated by dielectric heating, the temperature can be increased to a target temperature in about several hundred seconds. Thus, the temperature-increasing time is significantly reduced, and also, the time required for detection of an internal defect is greatly reduced. Moreover, advantageously, the temperature of the device can be quickly returned to its initial value, since the heating is local and spontaneous.
According to the present invention, the piezoelectric ceramic device is heated from inside using the dielectric heating by applying the high frequency signal of a high level. It requires about 400 msec. for inspecting the defects. Then, at the same time of this internal heating, when the piezoelectric ceramic device is heated externally, the measuring time can be shortened further, because the piezoelectric ceramic device can be heated to the predetermined temperature quickly. So, it is preferable to combine the internal heating and the external heating in order to reduce the measuring time.
In the internal heating (dielectric heating), the piezoelectric ceramic device generates heat at portions of vibration electrodes and the generated heat is transmitted to the outer peripheral portions. On the other hand, in the external heating, the generated heat of the piezoelectric ceramic device is transmitted from the outer peripheral portions to the inside. Thus, when the internal heating and the external heating are used at the same time, not only that the measuring time can be reduced, but also that the piezoelectric ceramic device can be heated to increase the temperature uniformly over the device.
As a method of the external heating, there may be a method of using the convection, a method of using the radient heat, a method of using the transmitting heat, etc. In order to heat and increase the temperature in a short period and with a simple equipment, the transmitting heat method is preferable.
When the phase characteristic and the impedance characteristic of the piezoelectric ceramic device in the state of being heated are measured, the defects which could not be inspected at the normal temperature can be inspected. However, there are some piezoelectric ceramic device whose phase characteristic goes beyond the standard value area in accordance with the temperature increase and goes back in the standard value area after the internal defects are inspected when the temperature is further increased. Such piezoelectric ceramic devices are also defect products.
FIG. 5 shows the relationship between the temperature and the maximum phase angle xcex8max in the heating process when the piezoelectric ceramic device is heated externally.
In FIG. 5, G denotes a good average phase characteristic. NG1 and NG2 are phase characteristics of the defect products having internal defects. In the cases of NG1 and NG2, the phase characteristics go away from the standard characteristic (the characteristic of G) at a predetermined temperature (70 or 120), and the internal defects can be inspected. However, when the temperature is increased further, the phase characteristics thereof do not differ from the standard characteristic, the internal defects cannot be inspected.
FIG. 6 shows the relationship between the lapse time and the maximum phase angle xcex8max, when the piezoelectric ceramic device is dielectric heated.
As shown from FIG. 6, the defect is inspected at the lapse time of about 150 msec. and after that the characteristic comes back to the normal value in case of NG1. In NG2, the characteristic is normal before the lapse time of 300 msec. but after that the defect is inspected.
Note here that the piezoelectric ceramic devices of NG1 and NG2 in FIG. 6 are the same as NG1 and NG2 in FIG. 5.
Further, the reason why the state of the defects of NG1 is different from that of NG2 is assumed to be due to the differences in generating positions of micro cracks.
In this way, when the inspection is performed only at a predetermined temperature and at a predetermined lapse time, a defect of the piezoelectric ceramic device whose characteristic returns to the standard value cannot be inspected.
So, according to the present invention, at least one of the phase characteristic and the impedance characteristic of the piezoelectric ceramic device is measured at a plurality of different temperature inside the device. When the measurement is performed at the plurality of temperature values, the internal defects of the piezoelectric ceramic device, whose characteristic goes away from the standard value and returns to the standard value, can be inspected.
As for the temperature interval of measuring, referring to FIG. 5, it is preferable to set the interval less than 50.
According to the present invention, the characteristics are measured at different temperature values inside the piezoelectric ceramic device. However, when the piezoelectric ceramic device is internally heated by applying the high frequency signal, it would be difficult to inspect the internal temperature of the piezoelectric ceramic device directly. In such a case, the characteristics are measured at a plurality of lapse times after applying the high frequency signal which is higher than the standard level.
When higher level high frequency signal is applied, as shown in FIG. 4, the internal temperature of the device increases in accordance with the lapse time. Therefore, when the characteristics are measured at a plurality of lapse times, the internal defects of the piezoelectric ceramic device, whose defects can be only in a certain range of temperature, can be inspected securely. Further, when the measuring timing is determined by the lapse time, it is not necessary to inspect the internal temperature of the device, thereby simplifying the measurement.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the detailed description of preferred embodiments of the present invention with reference to the attached drawings.